a) Field of the Invention
The present invention relates to a focus detecting optical system for use with photographic cameras, video cameras, still video cameras and so on.
b) Description of the Prior Art
Among the active type focus detecting optical systems which are so adapted as to perform focus detection by receiving reflected infrared light with a plurality of optical systems having parallax while projecting infrared light to objects to be photographed and the passive type focus detecting optical systems which are so adapted as to perform focus detection by receiving images of objects to be photographed with a plurality of optical systems having parallax, there are known numerous conventional focus detecting optical systems which are composed separately from photographic lens systems, as typically exemplified by the optical system disclosed by Japanese Patent Preliminary Publication No. Sho 55-149007 shown in FIG. 1 and that disclosed by Japanese Utility Model Preliminary Publication No. Sho 57-43409 shown in FIG. 2.
Each of these focus detecting optical systems is so designed as to detect a distance to an object to be photographed by detecting positional deviation between two images formed with two imaging optical systems which have the same composition and arranged with parallax. In FIG. 1, a first imaging optical system comprises a positive lens component 1 arranged in an optical path which is bent by a reflecting mirror 2 and a reflecting surfaces 3 so that the lens component 1 forms an image of the object to be photographed on a photoelectric converter element 7 such as a CCD (Charge Coupled Device) array or PSD (Semiconductor Position Sensing Device). A second image optical system consisting of a reflecting mirror 5, a reflecting surface 6 and a positive lens conpoment 4, i.e., having the same composition as that of the first imaging optical system, is arranged symmetrically with the first imaging optical system on the paper surface. In FIG. 2, the optical elements described above are formed integrally as a prism 8 which has a surface of incidence 1 functioning as the positive lens component as well as reflecting surfaces 2 and 3 designed as totally reflecting surfaces. The focus detecting optical system shown in FIG. 1 or FIG. 2 is of the so-called passive type wherein each of the first and second imaging optical systems forms an image while receiving light emitted from an object to be photographed. The so-called active type focus detecting optical system is adapted to form a light spot on the photoelectric converter element 7 while projecting light from a light emitting element such as a light emitting diode arranged at the location of P shown in the drawing on a photoelectric converter element 7 through the prism 8, in the example shown in FIG. 2, and condensing the light reflected by the object to be photographed on the photoelectric converter element 7 through the other optical system 9. The passive type focus detecting optical system calculates a distance to the object to be photographed by detecting a spacing between the two images, whereas the active type focus detecting optical system calculates the distance to the object by detecting a position of the light spot formed on the photoelectric converter element 7.
FIG. 3 and FIG. 4 illustrate, on an enlarged scale, one of the imaging optical systems of the focus detecting optical system shown in FIG. 2. In these drawings, the reference symbols a and b represent rays emitted from an object on which the photographic lens system is to be focused, with the symbol a corresponding to the upper marginal ray and the symbol b responding to the lower marginal ray. The reference symbol O designates the optical axis. Further, the reference symbol c denotes a stray light constituting a cause for ghost typically represented in the drawings by a ray on the meridional surface which is incident on an imaginary surface S placed on the surface of incidence of the positive lens component 4 at an angle of incidence of 89.degree. and at a location 3 mm above the optical axis. The reference numeral 10 represents the light exit surface of the prism 9 and the reference numeral 7a designates the light receiving surface of the photoelectric converter element 7. In the example shown in FIG. 3, a bonding agent 11 is filled between the light exit surface 10 and the light receiving surface 7a for fixing the photoelectric converter element 7 and preventing foreign matters from entering therebetween, whereas no substance is filled and an air layer is formed between the light exit surface 10 and the light receiving surface 7a in the optical system shown in FIG. 4. In the case of FIG. 3, the stray light c travels from the positive lens component 4 into the prism 9, emerges from the light exit surface 10 and attains directly to the light receiving surface 7 a without falling on the first reflecting surface 5 or the second reflecting surface 6. In the case of FIG. 4, the stray light c travels from the positive lens component 4 into the prism 9, is totally reflected by the light exit surface 10 and then reflected by the second reflecting surface 6, transmits through the light exit surface 10 and attains to the light receiving surface 7a. As is clear from the foregoing description, each of the optical systems shown in FIG. 3 and FIG. 4 has a defect that it allows the stray light c coming from an object other than that to be photographed to attain on the light receiving surface 7a, thereby constituting a cause for erroneous distance measurements.
Further, when a stop having an aperture diameter of 2 mm, for example, is arranged right before the surface of incidence of the positive lens component 4 or 1 in a position symmetrical with regard to the optical axis as shown in FIG. 5 and FIG. 6 which are enlarged partical sectional views like those illustrated in FIG. 3 and FIG. 4, the lower marginal ray having a height of incidence of -1 mm on the stop surface, out of the paraxial rays, is not totally reflected by the first reflecting surface 5, but transmits therethrough. Furthermore, rays having heights of incidence -0.5 mm and -1 mm on the stop surface contained in a light bundle to be incident on an offaxial location having an image height of 0.3 mm are not totally reflected by the first reflecting surface 5, but transmit therethrough since the stop having the aperture diameter of 2 mm is arranged symmetrically with regard to the optical axis. Accordingly, each of the optical systems illustrated in FIG. 5 and FIG. 6 poses a problem that it is incapable of leading all of the required rays to the light receiving surface 7a of the photoelectric converter element 7, thereby making it impossible to maintain the minimum light amount required for focusing so far as the first reflecting surface 5 is not coated with a reflecting substance such as aluminum. Moreover, as is understood from the fact that the number of the reflected rays is different between FIG. 5 and FIG. 6, the amount of light is different between the paraxial ray and the offaxial ray due to the nonuniform total reflection, thereby making distribution of brightness nonuniform on the light receiving surface 7a. Accordingly, the optical system poses a problem that light amount is reduced and distance measurement is erroneous more frequently for focusing the photographic lens system on an object located at a more offaxial position or a shorter distance. Coating the first reflecting surface 5 with a reflective substance such as aluminum will undesirably enhance manufacturing cost of the optical system.
In addition, the above-described focus detecting optical systems have field angles far narrower than those of the photographic lens systems to be used therewith, thereby posing a problem that the optical system is incapable of measuring distance within a broad range covering the entire photographing image surface. For this reason, the conventional focus detecting optical systems posed another problem that, when an image of an object to be photographed is not located at the central portion of the image surface, it is required to locate the image within the field angles of the focus detecting optical systems and lock the focusing system before performing framing, thereby retarding timing of shutter release of missing optimum shutter release chance. When the field angles of the focus detecting optical systems are simply made wider than that currently available, the first and second imaging optical systems will aggravate distortion and other aberrations, thereby making it impossible to maintain the correct spacing between the images or form a correct spot image.
Out of the conventional focus detecting optical systems described above, the optical system illustrated in FIG. 1 comprises a large number of optical elements or parts, thereby posing a problem that it requires high manufacturing cost and adjustments for matching the optical elements with one another, which further enhance manufacturing cost of the focus detecting optical system.